This invention relates to switched reluctance machines, and more particularly to a switched reluctance machine utilizing soft chopping to regulate the current, especially when the switched reluctance generator operates at low speed.
The continued advances in high-power switching semiconductors and control electronics have enabled the use of switched reluctance generators, which have been used extensively in motor applications in the past, to be increasingly exploited for the generation of electrical energy. The use of switched reluctance generators in such applications is highly desirable as the generators are simple and rugged due in part to the winding-free, magnet-free brushless construction of the salient pole rotor. This construction permits the use of the switched reluctance machine at high speeds and under harsh environmental conditions. Also, since the rotor lacks windings and magnets, it generally costs less than a wound or permanent magnet rotor.
A diagram of a switched reluctance machine, together with one phase winding and the associated power converter components, is shown in FIG. 1. Each phase winding of the switched reluctance machine 10 comprises two serially-connected coils (for example, coils 12 and 14) wound around diametrically opposed stator poles (for example poles 16 and 18). Torque is produced in the switched reluctance machine 10 by the tendency of the nearest rotor pole pair to move to a minimum reluctance position with respect to the excited stator pole pair. The magnitude and direction of the produced torque is determined by the magnitude of the exciting phase current pulses and the placement of these pulses with respect to rotor position. Ideally, the torque generated by an unsaturated switched reluctance machine is
Te(I,xcex8)=xc2xdI2(dL(xcex8)/dxcex8),
where I is the phase current, L is the phase inductance and xcex8 is the rotor angle. Note that the torque direction is independent of the sign of the current so the phase current can be unidirectional. Also, the sign of the torque is determined by the placement of the phase current pulse relative to the change of phase inductance, dL(xcex8)/dxcex8.
FIG. 2 illustrates an idealized example of the placement of the current pulses for torque and electrical power generation in the switched reluctance machine 10. Specifically, FIG. 2 illustrates the idealized phase inductance variation as a function of rotor angle (xcex8), the motoring current and the generating current, both also as a function of the rotor angle xcex8.
Returning to FIG. 1, each of the rotor poles is identified by a reference character 20. The six stator pole pairs are identified by reference characters 16, 18 and 22. When the stator pole 18 is not aligned with any of the rotor poles 20, the inductance there between is at its minimum value, as shown by the horizontal segment of the FIG. 2A inductance curve. As the rotor angle changes, the stator pole 18 begins to overlap the rotor pole 20, and the inductance rises and reaches a maximum value when the stator pole 18 is aligned with the rotor pole 20. Maximum inductance is illustrated in FIG. 2A by the vertical line bearing reference character 30. For motoring operation, the current is supplied to the diametrically opposed stator poles 16 and 18 via the windings 12 and 14, respectively, during the period when the inductance is increasing and the rotor poles 20 are approaching the stator poles 16 and 18. The motoring current is shown in FIG. 2B. Since the inductance is increasing in this region, the torque produced acts in the direction of rotor rotation, thus producing positive torque.
To generate electrical power, current must be supplied during the period when the inductance is decreasing as the rotor poles 20 pull away from the stator poles 16 and 18. See FIG. 2C. Since the phase inductance is decreasing in this region, the torque opposes rotor motion. The work done by the system to pull the stator and rotor poles apart is returned as energy to the DC bus, which also supplies the motoring and the generating current. Ideally, in the generating mode, the phase current should be provided in the region where the phase inductance is decreasing, as shown in FIG. 2C. However, given the back-EMF experienced by the switched reluctance machine 10, the phase current should be provided several degrees before the maximum phase inductance position is reached. This assures that sufficient current is available in the phase windings 12 and 14, for example, when the rotor poles 20 enter the region where the phase inductance begins to decrease.
Thus, the switched reluctance machine operates both as a motor and as a generator. The inductance of each phase winding (for example, the coils 12 and 14 of FIG. 1 comprise one phase winding) varies according to the degree of overlap between the stator poles 16 and 18 and the rotor poles 20 as the latter rotate. If current is supplied while the winding inductance is increasing (i.e., the degree of overlap is increasing) then the magnetic force on the rotor poles 20 tends to increase the degree of overlap by creating a positive torque. This physical phenomena is the basis for the motoring operation of the switched reluctance machine 10.
If current is applied to the coils 16 and 18 while the winding inductance is decreasing (i.e., the degree of overlap between the stator poles 16 and 18 and the rotor poles 20 is decreasing) then the resulting magnetic force opposes further separation of the rotor poles 20 and the stator poles 16 and 18. This separation acting against the magnetic force demands an input of mechanical energy to the rotor, which is in turn converted by the switched reluctance machine 10 into electrical energy in the form of an increasing winding current. This current reaches its maximum value when the inductance is high and as a result the opposing magnetic force (and therefore the generated current) is large during separation between the stator poles 16 and 18 and the rotor poles 20.
The switched reluctance machine 10 illustrated in FIG. 1 includes the stator poles 16 and 18 plus four additional stator poles 22. The FIG. 1 embodiment also includes four rotor poles 20, and is thus referred to as having a 6/4 topology (six stator poles and four rotor poles). As is recognized by one skilled in the art, a different topology can be utilized with corresponding changes in the controlling mechanism associated with the present invention (to be described herein below) without departing from the scope of the invention.
To allow rotation of the rotor poles 20, a small air gap 24 exists between the outer periphery of the rotor poles 20 and the inner periphery of the stator poles 16, 18 and 22. In one embodiment, this air gap is approximately 0.25 mm, but may vary due to machining and manufacturing tolerances or by design depending on the desired characteristics of the switched reluctance machine 10. Since a switched reluctance machine operates in accordance with the changing inductance between the rotor and stator poles, a slight change in the air gap has a significant impact on performance characteristics.
A simplified schematic of the control components associated with the phase windings 12 and 14 for providing commutation to the switched reluctance machine 10 is also illustrated in FIG. 1. A series connection of switch 32 and a diode 34 is connected across the DC bus 33, with the anode terminal of the diode 34 connected to the negative voltage of the DC bus 33. A series connection of a diode 36 and a switch 38 is also connected across the DC bus 33, with the cathode terminal of the diode 36 connected to the positive voltage. Note that the windings 12 and 14 are serially connected between the junction of the switch 32 and the cathode terminal of the diode 34 and the junction of the switch 38 and the anode terminal of the diode 36.
A schematic representation of the switches 32 and 38, the diodes 34 and 36 and the phase windings 12 and 14 is also illustrated in FIGS. 3A and 3B. The commutation approach described in conjunction with FIGS. 3A and 3B is illustrated in FIGS. 4A and 4B, which is somewhat more complex than the commutation approach illustrated in FIG. 2. FIG. 4B illustrates an example of single pulse operation where the current fed back to the bus 33 is not regulated. As the rotor rotates, the inductance of the stator windings (for instance the stator windings 12 and 14) varies as the salient rotor poles 20 come into and out of alignment with the stator poles 16, 18 and 22. The inductance variation is illustrated by a trace 39 of FIG. 4A. As the rotor poles 20 move toward alignment with the stator poles 16 and 18, the switches 32 and 38 close so that current flowing from the DC bus 33 energizes the stator windings 12 and 14. The arrows 40 and 42 indicate the current direction. This current is supplied beginning at a turn-on angle, as the rotor poles approach alignment with the stator poles, as indicated by the vertical line 44 in FIG. 4A, which identifies the current turn-on time.
Once both the switches 32 and 38 are closed, the current through the stator coils 12 and 14 increases, as indicated by the trace 46 of FIG. 4B, and the rotor poles 20 are attracted to the stator poles 16 and 18. At the vertical line 48, the rotor and the stator poles are aligned and the inductance therefore peaks. Beyond this point, as the rotor poles 20 continue to rotate, the inductance decreases, causing the back-EMF in the coils 12 and 14 to become positive. Note that the back-EMF is negative up to the point where the inductance begins to decrease. Since the back-EMF is now positive, it is added to the DC bus voltage and thus the current increases at a faster rate. As the inductance decreases, the current increases rapidly and the back-EMF also increases, until eventually the back-EMF exceeds the DC bus voltage. Once the current exceeds the upper current limit (IHI) at the vertical line 50 of the trace 46, both the switches 32 and 38 open, as shown in FIG. 3B. Now current is returned to the DC bus 33 through the diodes 34 and 36.
Due to the decreasing inductance during this segment of the cycle, the current delivered to the bus continues to increase. However, once the rotor and stator poles are not overlapped, beginning at the vertical line 52, the inductance reaches its minimum value, where it remains until the rotor and stator poles begin to approach alignment again during the next electrical cycle. During this segment, the current delivered to the bus 33 decays to zero. Because more current is generated during the decrease in inductance (as the rotor pole pulls away from the stator pole) than is required to be supplied by the DC bus 33, a net generation of electrical power occurs. The switches 32 and 38 can be implemented with insulated-gate bipolar transistors, metal-oxide-semiconductor-controlled thyristors and static induction transistors, as well as other power switching devices known in the art.
The prior art switched reluctance machine uses hard chopping to regulate the current during the generating mode. Hard chopping refers to the use of only the positive voltage and the negative voltage to regulate the current in a winding of a switched reluctance machine. The positive voltage is applied when both the switch connected to the positive bus and the switch connected to the negative bus are turned on or closed to energize the winding. See FIG. 3A. The negative voltage is applied when both the positive and negative bus connected switches are turned off or opened to de-energize the winding. As a result, the induction current free wheels through the diode connected to the positive bus voltage and the diode connected to the negative bus voltage. However, the efficiency of this technique is poor and there is considerable ripple in the phase current, because the current waveform has a significantly higher switching frequency then the soft chopping approach of the present invention.
In accordance with the teachings of the present invention, a soft chopping technique during the current generation phase reduces the current ripple. It is known that a ripple in the output current produces torque ripple in the switched reluctance machine, which in turn generates noise. Thus, a switched reluctance machine constructed according to the teachings of the present invention reduces this noise as well as increases the efficiency of the power conversion process by virtue of the decreased ripple in the output current. In addition, less power is wasted by recirculation of energy through the power converter, and the switched reluctance machine. Less ripple in the output current also results in an increase in the generated power and a reduction in the current filtering requirements. The soft chopping technique of the present invention applies a zero voltage, a positive voltage and a negative voltage to the phase winding to regulate the current in the winding. The positive and negative voltages are applied in a manner similar to the application in the hard chopping process. The zero voltage is applied to the phase winding by essentially disconnecting the phase winding from the positive and negative bus voltages and allowing the current to freewheel through a diode, a switch and the phase winding.
The soft chopping technique of the present invention is implemented through hysteresis current control during the period when the inductance of the switched reluctance machine phase winding is decreasing. Although the process of the present invention is especially advantageous during low speed operation of the switched reluctance machine, the teachings of the present invention can be employed at any operating speed.